Particles made from inorganic hydrophilic materials and useful as carriers for electrographic toners are prepared by numerous methods, probably the most primitive of which is grinding the bulk material to a suitable particle size and/or particle size distribution. The disadvantages of this process are widely known. Initially, the particles are irregularly shaped after grinding and may not be suitable for their intended purpose. Second, some materials are not amendable to grinding due to their physical properties. Third, the particle size distribution produced by grinding is broad.
In typical copying processes, toner and/or carrier particles are subject to electrostatic and other forces that affect the particles differently depending on their size. Therefore, in order to obtain high quality copies, it is preferred that toner and/or carrier particles have a controlled morphology and narrow size distribution. Past methods of attaining particles of the desired size and size distribution include sizing the toner and/or carrier particles by passing the ground polymer through progressively smaller sieves to classify the particles by size. Recently, to avoid this expensive and time consuming process, new methods have been developed to produce particles having a controlled size distribution and morphology.
Other methods of forming particles from hydrophilic material include dispersing the hydrophilic material in,a liquid dispersant. Examples include rotary spray drying, ultrasonic spray drying, ultrasonic dispersion in an immiscible liquid, or shear dispersion in an immiscible liquid. The size distribution and morphology of the particles formed by these processes is dependent upon the manner in which the dispersion is broken into droplets and the dispersant removed. Typically, the resultant particle size of the material is directly related to the droplet size formed by the processing equipment. These methods all possess the disadvantage of forming particles with a generally broad particle size distribution.
A method commonly known as "limited coalescence" has been applied to solutions or dispersions of hydrophobic, polymeric materials in U.S. Pat. Nos. 4,833,060 to Nair, et al. and 4,965,131 to Nair, et al. (collectively, the "Nair processes"). These limited coalescence processes have the advantage of controlling the final droplet size independent of the means by which the solution or dispersion is broken into droplets. In limited coalescence methods, the solution or dispersion is broken into droplets much smaller than the desired final droplet size by shearing the dispersion of hydrophobic material in an immiscible suspending liquid, usually water, containing a colloidal stabilizer. The droplets coalesce upon the removal of shearing forces and the 5 colloidal stabilizer preferentially adsorbs at the interface between the immiscible liquids and limits the coalescence of the droplets. Therefore, the amount of stabilizer, not the type of shear, determines the final droplet size. It has been found that narrow particle size distributions are generated when the solution or dispersion is broken into droplets substantially smaller than the final droplet size.
To be effective, the colloidal stabilizer must be well dispersed in the suspending liquid. Electrostatic stabilization of the colloid through ionization of groups on the colloid or surfactants adsorbed on the colloid is effective to disperse the colloid in polar suspending liquids. Current limited coalescence methods which employ electrostatic stabilization of the colloid are, however, restricted to systems where the suspending liquid is more polar than the dispersant.
The Nair processes are also limited to the formation of particles from hydrophobic materials, such as polymeric materials useful as electrographic toners. U.S. Pat. No. 4,506,062 to Flesher, et al. ("Flesher") discloses an inverse suspension polymerization process in which an aqueous solution of polymerizable material is dispersed as droplets in a water immiscible liquid. A dispersion of suspension stabilizer is added to the water-immiscible liquid before the polymerization material is dispersed with the liquid. The material is polymerized in the droplets to form a dispersion of solidified particles in the non-aqueous liquid. Flesher relies on solvation of the stabilizer by the water-immiscible liquid to obtain a uniform dispersion of the stabilizer in the suspension. The stabilizer is attracted to the water droplet interface by ionic groups of the stabilizer and fixed there by an oppositely charged costabilizer which is insoluble in the non-aqueous liquid. Flesher, essentially, requires the dissolved molecules or solvated colloids of stabilizer to be in equilibrium with the water-immiscible liquid before the addition of the water phase. This limits the variety of stabilizers that can be used in the Flesher process because a stabilizer's ionizable groups must ionize upon the addition of the aqueous phase so that the stabilizer is attracted to the water phase droplets. Thus, even if a stabilizer is sufficiently hydrophilic in the absence of water to work as a suspension stabilizer in the Flesher process, surfactants are required to disperse them in the hydrophobic phase. Surfactants, however, typically interfere with the stabilization of the droplets.
In addition, Flesher addresses only suspension polymerization of hydrophilic materials to form particles having a narrow size distribution and controlled morphology. In suspension polymerization processes, the polymerization process itself solidifies the particles from the coalesced droplets and there is no solvent to remove. Therefore, there continues to be a need for methods of forming particles having a controlled particle size distribution and morphology from dispersion of inorganic hydrophilic materials.